Conventionally, cords for rubber reinforcement have been proposed.
For example, JP2001-114906A discloses a cord for rubber reinforcement that excels in bending fatigue resistance by the construction in which primary twist strands are used as a core member (inner layer) and a side member (outer layer).
JP2004-11076A discloses a cord for rubber reinforcement that excels in bending fatigue resistance and dimensional stability by the construction in which strands having different primary twist directions are used as a core member and a side member.
JP10 (1998)-141445A, JP9 (1997)-42382A, JP1 (1989)-213478A, and JP59 (1984)-19744A disclose cords for rubber reinforcement in which the number of primary twists and final twists of strands is limited to improve bending fatigue resistance. Further, JP7 (1995)-144731A, JP10 (1998)-291618A, JP2005-8069A, and JP2005-22455A disclose cords for rubber reinforcement in which the number of twists and the direction of twist of the strands are limited.
A drawback of conventional cords for rubber reinforcement, however, is that, when the cord is bent, a shear force causes a crack in the adhesive layer (for example, RFL layer) that binds the primary twist threads in a cord and eventually destroys the cord from the point of cracking. In other words, the conventional cords for rubber reinforcement with the limited number of twists and the limited twist direction do not have sufficient bending fatigue resistance.
When the cord is bent repeatedly, the crack first occurs in the adhesive layer between the primary twist threads. The crack changes the overall balance of stress in the cord, creating strong stress that locally concentrates on each primary twist thread. The concentration of stress breaks the strands making up the primary twist threads and eventually destroys the entire cord.
One effective way to reduce the shear force acting on the adhesive layer is to increase the number of final twists. However, simply increasing the number of final twists produces a cord with poor dimensional stability that easily stretches, or leads to weak tensile strength.